In manufacturing electronic components such as electronic circuits, circuit boards, resistors, capacitors and IC packages, conductive metal powder is used to form conductor films and electrodes. The characteristics and properties/conditions required for this kind of metal powder include low impurity, fine powder having an average particle diameter of about 0.01 to 10 μm, uniformity in particle shape and particle diameter, little cohesion, excellent dispersibility in paste and excellent crystallinity.
Recently, conductor films and electrodes have been thinner and fine-pitched as electronic components and circuit boards have reduced in size, so that finer spherical metal powder having high crystallinity has been demanded.
As one of methods for producing such fine metal powder, there are known plasma devices which, after melting and evaporating a metal starting material in a reaction vessel by utilizing plasma, cool and condense the metal vapor to produce a large number of metal nuclei, and grow these to obtain metal powder. (Refer to Patent Literatures 1 and 2.) These plasma devices condense the metal vapor in a gas phase, thereby capable of producing fine spherical metal particles having high crystallinity and low impurity.
Further, these plasma devices each have a long cooling tube and cool a carrier gas containing the metal vapor through a plurality of steps. For example, the plasma device described in Patent Literature 1 is provided with a first cooling unit which cools the carrier gas by directly mixing the carrier gas with a preheated hot gas and a second cooling unit which thereafter cools the carrier gas by directly mixing the carrier gas with a room-temperature cooling gas. Further, the plasma device described in Patent Literature 2 is provided with an indirect cooling section (first cooling unit) which cools the carrier gas by circulating a cooling fluid around a tubular body without making the fluid directly contact the carrier gas and a direct cooling section (second cooling unit) which thereafter cools the carrier gas by directly mixing the carrier gas with a cooling fluid.
Because the latter adopts indirect cooling in which radiative cooling is dominant, as compared with another plasma device in which conductive or convective cooling is dominant, from the metal vapor, the metal nuclei (hereinafter simply referred to as “nuclei”) are uniformly produced, grown and crystallized, and the metal powder with a particle diameter and a particle size distribution controlled can be obtained, in particular.